1. Field of the Invention
The invention relates to a protector assembly to be mounted to a drill pipe string extending along the bore of a well, for example an oil and/or gas well.
2. Background of the Invention
When drilling the bore hole of an oil and/or gas well, the friction between the drill pipe string and the open hole or the casing lining the hole limits the reach of the drilling action in particular if the drill pipe string is subject to lateral deflection, for example when drilling along a curved or angled path. Contact between the drill pipe string and the bore hole or its casing creates frictional torque and drag and limits the weight that can be put on the drilling bit and the effective control of the weight. In addition, the drill pipe string is subjected to increased shock and abrasion with a likelihood of damage to the drill pipe string or the casing. Shocks and vibrations occurring at the drill pipe string reduce the accuracy of the well bore.
To reduce friction between a drill pipe string and the bore hole or its casing, it is known from GB-A-2 400 124 to provide the drill pipe string with a plurality of protector sleeves rotatably mounted each on a different section of the drill pipe string. Each protector sleeve has an outer diameter greater than the outer diameter of joints connecting adjacent drill pipe sections and is rotatably seated on the outer diameter of the drill pipe section axially between a pair of thrust bearings axially fixing the protector sleeve with respect to the drill pipe section.
During drilling operation, pressurized drilling fluid flows through the drill pipe string down to the drilling bit and returns to the surface level through the annular space (annulus) between the drill pipe string and the bore hole or its casing, respectively. To reduce the friction between the protector sleeve and the drill pipe section, the radially inner surface of the prior art protector sleeve is provided with a plurality of longitudinal grooves extending along the protector sleeve to allow the drilling fluid to bypass the protector sleeve and to thus “lubricate” the protector sleeve when rotating relatively to the drill pipe section.
The drilling fluid flowing to the surface level of the well contains abrasive mud which limits the useful life of the protector sleeve and limits the degree of reduction of friction since it is hardly possible to make fully use of the friction reducing capacities of a fluid bearing.
Similar drill pipe protector assemblies are known from WO 95/10 685 A and EP 0 439 279 A1. From WO 95/10 685 A and U.S. Pat. No. 5,740,862 it is further known to seal a protector sleeve relatively to the outer diameter of the drill pipe section rotatably carrying the protector sleeve and to lubricate the protector sleeve by oil or grease included in an annular gap radially between the drill pipe and the protector sleeve.